How To Add Swap on CentOS 6

About Linux Swapping

Linux RAM is composed of chunks of memory called pages. To free up pages of RAM, a “linux swap” can occur and a page of memory is copied from the RAM to preconfigured space on the hard disk. Linux swaps allow a system to harness more memory than was originally physically available.

However, swapping does have disadvantages. Because hard disks have a much slower memory than RAM, server performance may slow down considerably. Additionally, swap thrashing can begin to take place if the system gets swamped from too many files being swapped in and out.

Note

Although swap is generally recommended for systems utilizing traditional spinning hard drives, using swap with SSDs can cause issues with hardware degradation over time. Due to this consideration, we do not recommend enabling swap on DigitalOcean or any other provider that utilizes SSD storage. Doing so can impact the reliability of the underlying hardware for you and your neighbors.

If you need to improve the performance of your server, we recommend upgrading your Droplet. This will lead to better results in general and will decrease the likelihood of contributing to hardware issues that can affect your service.

Check for Swap Space

Before we proceed to set up a swap file, we need to check if any swap files have been enabled by looking at the summary of swap usage.

swapon -s

If nothing is returned, the summary is empty and no swap file exists.

Check the File System

After we know that we do not have a swap file enabled, we can check how much space we have on the server with the df command. The swap file will take 512MB— since we are only using up about 7% of the /dev/hda, we can proceed.

Create and Enable the Swap File

“of=/swapfile” designates the file’s name. In this case the name is swapfile.

Subsequently we are going to prepare the swap file by creating a linux swap area:

sudo mkswap /swapfile

The results display:

Setting up swapspace version 1, size = 536866 kB

Finish up by activating the swap file:

sudo swapon /swapfile

You will then be able to see the new swap file when you view the swap summary.

swapon -s
Filename Type Size Used Priority
/swapfile file 524280 0 -1

This file will last on the server until the machine reboots. You can ensure that the swap is permanent by adding it to the fstab file.

Open up the file:

sudo nano /etc/fstab

Paste in the following line:

/swapfile swap swap defaults 0 0

To prevent the file from being world-readable, you should set up the correct permissions on the swap file:

chown root:root /swapfile
chmod 0600 /swapfile

How To Configure Swappiness

The operating system kernel can adjust how often it relies on swap through a configuration parameter known as swappiness.

To find the current swappiness settings, type:

cat /proc/sys/vm/swappiness

60

Swapiness can be a value from 0 to 100. Swappiness near 100 means that the operating system will swap often and usually, too soon. Although swap provides extra resources, RAM is much faster than swap space. Any time something is moved from RAM to swap, it slows down.

A swappiness value of 0 means that the operating will only rely on swap when it absolutely needs to. We can adjust the swappiness with the sysctl command:

sysctl vm.swappiness=10

vm.swappiness=10

If we check the system swappiness again, we can confirm that the setting was applied:

cat /proc/sys/vm/swappiness

10

To make your VPS automatically apply this setting every time it boots up, you can add the setting to the /etc/sysctl.conf file:

sudo nano /etc/sysctl.conf

# Search for the vm.swappiness setting. Uncomment and change it as necessary.
vm.swappiness=10